Meltspun technologies, which are known in the art to include the spunbond and meltblown processes, manage the flow of process gases, such as air, and polymeric material simultaneously through a die body to effect the formation of the polymeric material into continuous or discontinuous fiber. In most known configurations of meltblowing nozzles, hot air is provided through a passageway formed on each side of a die tip. The hot air heats the die and thus prevents the die from freezing as the molten polymer exits and cools. In this way the die is prevented from becoming clogged with solidifying polymer. In addition to heating the die body, the hot air, which is sometimes referred to as primary air, acts to draw, or attenuate the melt into elongated micro-sized filaments. In some cases, a secondary air source is further employed that impinges upon the drawn filaments so as to fragment and cool the filaments prior to being deposited on a collection surface. Typical meltblown fibers are known to consist of fiber diameters less than 10 microns.
More recently, methods of forming fibers with fiber diameters less than 1.0 micron, or 1000 nanometers, have been developed. These fibers are often referred to as ultra-fine fibers, sub-micron fibers, or nanofibers. Methods of producing nanofibers are known in the art and often make use of a plurality of multi-fluid nozzles, whereby an air source is supplied to an inner fluid passageway and a molten polymeric material is supplied to an outer annular passageway concentrically positioned about the inner passageway. While the physical properties of nanofiber webs are advantageous to a variety of nonwoven markets, commercial products have only reached limited markets due to associated cost.
U.S. Pat. No. 5,260,003 and No. 5,114,631 to Nyssen, et al., both hereby incorporated by reference, describe a meltblowing process and device for manufacturing ultra-fine fibers and ultra-fine fiber mats from thermoplastic polymers with mean fiber diameters of 0.2-15 microns. Laval nozzles are utilized to accelerate the process gas to supersonic speed; however, the process as disclosed has been realized to be prohibitively expensive both in operating and equipment costs.
U.S. Pat. No. 6,382,526 and No. 6,520,425 to Reneker, et al., also both hereby incorporated by reference, disclose a method of making nanofiber by forcing fiber forming material concentrically around an inner annular passageway of pressurized gas. The gas impinges upon the fiber forming material in a gas jet space to shear the material into ultra-fine fibers. U.S. Pat. No. 4,536,361 to Torobin, incorporated herein by reference, teaches a similar nanofiber formation method wherein a coaxial blowing nozzle has an inner passageway to convey a blowing gas at a positive pressure to the inner surface of a liquid film material, and an outer passageway to convey the film material. An additional method for the formation of nanofibers is taught in U.S. Pat. No. 6,183,670 to Torobin, et al., which is hereby incorporated by reference.
Spacing of nozzles within the die body may be arranged such that material exiting the nozzle arrangement can be collected in a more uniform manner upon a forming surface. It has been recognized that a linear formation of equally spaced nozzles may result in a striping pattern that is visibly noticeable within the collected web. The stripes are found to reflect the spacing between adjacent nozzles. The striping effect seen in the web can further be described as “hills and valleys” whereby the “hills” exhibit a noticeably higher basis weight than that of the “valleys”. The industry may also refer to such basis weight inconsistencies as gauge bands.
U.S. Pat. Nos. 5,582,907 and 6,074,869, both incorporated herein by reference, address striping observed in meltblown webs by organizing nozzles into two linearly arranged parallel rows each having substantially equally spaced. Additionally, the two rows of nozzles are offset such that the nozzles are staggered in relationship to each other. Further, the staggered nozzles of the two rows are angled inward toward each other. In this fashion, each nozzle is utilizing a respective supply of primary process air, but lacks an ancillary air source to assist with web formation. These patents further assert external disruption of the polymeric material by an alternate gas source detracts from achievement of sufficient web uniformity.
A need remains for a process that can utilize multi-fluid openings for facilitating the distribution of molten polymer and a gas in the formation of nanofibers and further incorporates an ancillary gas source that assists with a uniform fiber collection across the width of the web.